1. Field of the Invention
The present invention relates to a transistor oscillating circuit having a circuit structure capable of reducing change in oscillating frequency due to change in power supply voltage.
2. Description of the Related Art
Conventionally, as a Colpitts transistor oscillating circuit for generating a radio frequency signal, a transistor oscillating circuit shown in FIG. 2 is known.
As shown in FIG. 2, the transistor oscillating circuit includes an oscillating circuit unit 21 having a resonance circuit connection node 21a, an oscillating signal output node 21b, and a power supply connection node 21c, and a resonance circuit 22 having an output node 22a and a control node 22b. In this case, the oscillating circuit unit 21 includes an oscillating transistor 23, coupling capacitors 24 and 25, feedback capacitors 26 and 27, an emitter load resistor 28, base bias resistors 29 and 30, and a bypass capacitor 31. The resonance circuit 22 includes λ/4 dielectric resonator 32, a varactor diode 33, a DC blocking capacitor 34, and a buffer inductor 35.
In addition, in the oscillating circuit unit 21, an emitter of the oscillating transistor 23 is connected to ground through the emitter load resistor 28, to the oscillating signal output node 21b through the coupling capacitor 25, to a base thereof through the feedback capacitor 26, and to a collector thereof through the feedback capacitor 27. The base of the oscillating transistor 23 is connected to the resonance circuit connection node 21a through the coupling capacitor 24, to a collector thereof through the base bias resistor 29, and to the ground through the base bias resistor 30. The collector of the oscillating transistor 23 is directly connected to the power supply connection node 21c and to the ground through the bypass capacitor 31. In this case, the resonance circuit connection node 21a is connected to the output node 22a of the resonance circuit 22, the oscillating signal output node 21b is connected to a next-stage buff amplifier (not shown), and the power supply connection node 21c is connected to a power supply terminal (no reference numeral) for receiving a power supply voltage Vcc.
In addition, in the resonance circuit 22, one end of the λ/4 dielectric resonator 32 is connected to the output node 22a and the other end thereof is connected to the ground. An anode of the varactor diode 33 is connected to the ground and a cathode thereof is connected to the output node 22a through the DC blocking capacitor 34 and to the control node 22b through the buffer inductor 35. In this case, the control node 22b is connected to a control terminal Vf for receiving a frequency control voltage.
The oscillating circuit unit 21 having the above-described structure is a Colpitts transistor oscillating circuit, in which an oscillating frequency is set by the feedback capacitors 26 and 27 connected between the base and the emitter of the oscillating transistor 23 and the resonance circuit 22 RF-connected between the base and the collector, and the oscillating signal output from the emitter of the oscillating transistor 23 is supplied to the oscillating signal output node 21b through the coupling capacitor 25.
In the conventional transistor oscillating circuit, the base bias resistors 29 and 30 of the oscillating transistor 23 are connected between the power supply connection node 21c and the ground in series and a divided voltage, which is generated in a connection point thereof, is supplied to the base of the oscillating transistor 23 as the base bias voltage Vb. In this case, when resistance values of the base bias resistors 29 and 30 are R1 and R2, respectively, a current amplifying ratio of the oscillating transistor 23 is sufficiently large, and the base current can be ignored, the base bias voltage Vb is set by Vb=Vcc×{R2/(R1+R2)}.
At this time, when a voltage between the collector and the base of the oscillating transistor 23 is Vcb, a voltage between the collector and the emitter thereof is Vce, and a junction voltage between the base and the emitter is 0.7 V, the base bias voltage is set byVcb=Vcc−Vb=Vcc×{R1/(R1+R2)}Vce=Vcb+0.7=Vcc×{R1/(R1+R2)1+0.7
As can be seen from above equations, when the power supply voltage Vcc is changed, the voltage Vcb between the collector and the base and the voltage Vce between the collector and the emitter of the oscillating transistor 23 are changed in correspondence with change in power supply voltage Vcc, an internal capacitance between the collector and the base and an internal capacitance between the collector and the emitter of the oscillating transistor 23 are changed depending on the above-described change, and the oscillating frequency of the oscillating transistor 23 is significantly changed.